1. Field of the Invention
This invention relates to telephone subscriber loops and, more particularly, to methodologies and concomitant systems for determining the composition of a subscriber loop from frequency domain measurements at the input of the loop.
2. Description of the Background Art
With the deployment of high-speed data transmission techniques on subscriber telephone loops, such as ISDN (Integrated Services Digital Network) and DSL (Digital Subscriber Loop), there has been a renewed interest in devising a technique for determining the composition of the loops from so-called single-ended measurements in order to qualify such loops for such high-speed digital transmission. A telephone subscriber loop typically connects a customer with a local telephone central office and is composed of lengths of copper cable such as, for example 26 gauge or 24 gauge cable. It is especially desirable to estimate the configuration of a loop from measurements made at the end of the loop terminating at the central office. For example, one might measure the complex input impedance at the input of the loop over a range of frequencies (generally referred to as the frequency domain or “swept-frequency approach”), or the time-domain echo at the input to the loop (generally referred to as the time domain or “time domain reflectometer” approach). From these measurements, the composition of the loop is estimated using identifiable characteristics in the response, such as peaks of return signals in the frequency domain or time intervals between peaks in the response signal. Moreover, based upon estimates of the loop configuration from these measurements, it is further possible to estimate the transmission characteristics of the loop to the customer end. It is desirable to effect such measurements using a single-ended approach at the central office so such tests can be automated.
One prior art reference that treats aspects of the loop composition problem is U.S. Pat. No. 3,904,839, dated Sep. 9, 1975, issued to Peoples and entitled “Loop Fault Locator”. The focus of '839 is on locating cable “faults”. In the past, a successful fault location program generally involved the following steps: fault sectionalization, fault localization, and fault pinpointing. Each step produced a more refined estimate to the location of a fault. The goal of fault sectionalization is that of locating the access point (e.g., terminal, splice, cross-connect box) nearest the fault from measurements at the central office. The fault localization activity uses measurements at the access point to further refine the estimate of the location of the fault. Finally, fault pinpointing involves “walking” the loop with loop test equipment to zero in on the fault.
The definition of “fault” is very broad in this context. Certain loop conditions are indeed faults, such as an open in one of the two conductors comprising a loop, or a short at some point in the loop. On the other hand, other conditions are truly not faults in the usual sense; for example, a gauge change (a cable of one gauge such as 26 gauge being spliced to a cable of another gauge such as 24 gauge) could be interpreted as a fault since there is an electrical discontinuity at the junction of the two gauges (that is, the two gauges have different primary or secondary electrical constants). However, such a cascade of gauges is actually designed into the loop. The smaller gauge such as 26 gauge is purposely placed closest to the central office to reduce congestion in conduits. Other, coarser gauges are used remotely from the central office to ensure the customer has sufficient current to operate the telephone or other customer premises equipment. The terminology is generalized herein so both actual faults and perturbations due to, for example, gauge changes or the end-of-loop are referred to as irregularities.
The technique of '839 uses a frequency domain approach. The input impedance phase derivative is measured across a range of several octaves above a specified starting frequency to produce a corresponding periodic function. Each irregularity (such as low resistance splice or a gauge change) produces an additive sinusoidal variation in the phase derivative as frequency is increased. The frequency of each sinusoidal variation is linearly related to the distance to the corresponding irregularity and, therefore, provides an estimate to the distance to the irregularity. The frequencies of the sinusoids are determined, using analog or digital means such as computer processing, from the maxima in a transformed function determined from the periodic swept-frequency function.
Numerous other loop functions are measurable at the input to the loop, including the magnitude of the input impedance, the real part of the input impedance, the imaginary part of the input impedance, the phase or phase derivative of the input impedance, and functions related to the return loss (a term of art wherein the input impedance is compared to a reference impedance) such as the real part, the imaginary part, and so forth as for the input impedance alone. The '839 reference uses the phase Wderivative of the input impedance because, empirically, it is the most sensitive indicator of the distance to irregularities.
There are known limitations on single-ended measurements made in either the time domain or frequency domain. For example, with the swept-frequency technique of '089, if two faults are closely spaced, then the power spectral peak of one fault can dominate or mask the power spectral peak of the other fault. This problem is characterized as one of “resolution”, that is, how far apart must irregularities be in order to mitigate interaction to find the associated peak of each irregularity. There is no ready answer to this; each loop must be evaluated separately due to the complexity of the interaction effects of various irregularities. Also, each irregularity gives rise to a multiplicity of sinusoidal terms that can mask even the fundamental sinusoidal frequency of another irregularity. This masking effect is not crucial if the intent is to merely “sectionalize” an irregularity. However, to qualify loops for ISDN or DSL deployment, the coarse results obtained by using the technique of '089 are typically not satisfactory.
Besides the limitations imposed by loop composition such as closely space irregularities, there is also the known problem of estimating peaks in a power spectrum which has been generated using data limited to a finite range of frequencies. This problem falls into the class of interval-limited time or frequency domain sampling. The limited amount of data is equivalent to truncating the complete frequency domain representation of the function being measured by a “weight function”. The default, and typical, weight function is a rectangular window. However, such a weight function severely distorts the peaks in the power spectrum because of the interaction of the slow decay of the weight function in the power spectral domain, that is, the transform domain. Improved resolution can be achieved if another weight window is used. Known examples of such weight windows are the raised-cosine, Hamming window, or Kaiser window. However, even with these “weight windows” there is still the possibility of distortion in the transform domain because of so-called “aliasing” wherein spectral components are interfered with by the decay (albeit more rapid than the rectangular window) of these weight functions in the transform domain.
The prior art is devoid of improved signal processing techniques that can refine the resolution of the swept-frequency measurements so as to further identify previously-masked peaks in the power spectrum. Whenever more peaks can be identified, the composition of the loop can be estimated with greater accuracy.